# I was wondering

1. Jan 8, 2004

### ALM

if the universe is expanding it means that the universe is moving. if something is moving it has direction and velocity. it has to be going somewhere, doesnt it? I think of the universe expanding in all directions and it has a velocity which is probably getting slower and slower because the further it gets from the original point (big bang) the more energy it looses. we may agree on that?... if we do then the universe is moving. but to move you need somewhere to go... it cant be nothingness (I think of nothing as that which we cant understand or see but that doesnt mean is not there). the other way to see it is that the universe itself is still and everything inside it is moving towards the limits of the universe.... I dont know... its difficult too understand the concept of universe and its even more dificult to understand its limits... but anyway I know there arent any questions here to solve. I just want to know if any of you have an opion. thanks
ALM

2. Jan 8, 2004

### Nereid

Staff Emeritus
The idea of an expanding universe, cf a static one, is quite hard to grasp.

As you say, we intuitively feel that if something is 'expanding', it - or parts of it - must also be 'moving'.

However, what's 'expanding' in the universe is space itself.

Think of a balloon being blown up. It starts small and gets bigger and bigger. Two blobs on the balloon will get further apart as the balloon gets bigger, even though neither blob is actually moving.

The universe is like a balloon, except in 3D; we (Earth, solar system, Milky Way) are like a blob.

The tricky part is making your mind 'see' that our 3D universe is like the 2D surface of the balloon and that there's no 'fourth dimension' that the universe is expanding into (unlike the balloon expanding into the third spatial dimension).

Perhaps some other posters can recommend a good text which explains these difficult concepts more clearly than I can.

3. Jan 8, 2004

### marcus

ALM your post suggests you want a sampling of other people's viewpoints.
Nereid, you explain it just fine. Lots wd agree with the image you gave of the balloon.

My first astronomy teacher used the image of raisinbread dough rising. The raisins are galaxies and they are all standing still (not swimming thru the dough) and all the same the raisins (galaxies) keep getting farther and farther apart.

From each raisin's point of view it looks like the others are running away from it---and the farther away the other raisin is the faster it seems to receed.

But none of them are moving. It is only that the distances between them are all constantly increasing.

That was how my beginning astro prof suggested we think of it.

whether the expansion is speeding up or slowing down depends on
a certain differential equation called Friedmann equation and the
average energy density that gets plugged into it---right now it looks to the astronomers as if the expansion of space is speeding up.
but that is more of a technical detail, not to worry about such things at first but instead just get used to the idea of the expansion itself----the fact that distances between stationary non-moving things can increase. And I mean distances not between nearby stars but between remote galaxies (far enough apart that their attraction for each other is negligible). All of these distances increase by a certain percentage each year. the percentage is different year by year, but in a given year each distance increases by the same tiny percentage, or the same "fractional amount" since that sounds more scientific than saying "tiny percentage".

I will tell you what the annual percentage growth or fractional growth amount is.

It is one part in 14 billion.
Or the fraction 1/14,000,000,000.

If there is some galaxy that at this minute is 14 billion LY away from us (by the usual cosmologist's distance measure) then
this tiny fraction of that is simply ONE lightyear. And so I am saying that in one year the distance to that galaxy will increase by one lightyear.

And it is proportional, so if there is some other galaxy that is right now 28 billion LY away, then in one year the distance to it will increase by 2 LY.

That is, next year it will be 28,000,000,002 LY from us.

This is hard enough to assimilate, I think. Nereid might agree too.
there is no harm in continuing to ask questions until you get used to it and the shock is over. there are unintuitive aspects to this
that you may discover by thinking it over. Like, this picture does not necessarily mean that it all started at a point. Space could always have been infinite, and yet expanding. I remember finding that hard to grasp when I first confronted it, so you might possibly find it so as well. It COULD have begun at a point but it might not have.
The raisinbread dough could have begun as an infinite loaf, extending endlessly in all directions and with its raisins very close together. Or it could have begun at one extremely raisiny point of departure. Either way the thought is positively hair-raising.